Ten out of 26 beef samples tested last month contained the toxic substance sulfur dioxide, believed to have been used as a preservative to make the meat look redder and fresher.

No preservative is allowed to be used in meat but the Food and Environmental Hygiene Department, which conducted the tests, said one sample contained as much as 5,500 parts per million of sulfur dioxide.

The biggest concentration was found in a sample from the Wah Fung stall in Sham Shui Po's Pei Ho Street Market - one of 22 outlets checked. A sample from the Wing Shun stall in Ma On Shan's Sunshine City Plaza had 3,200 parts per million.

A sample from another stall in the Pei Ho Street Market contained 2,500 parts per million. Levels in samples from the Hung Hom, Sha Tin and Shek Wu Hui markets ranged from 17 to 830 parts per million.

Use of preservatives, including sulfur dioxide, in fresh, chilled or frozen meat is banned under the Preservatives in Food Regulation. Offenders are liable to a maximum fine of HK$50,000 and six months' jail.

People allergic to the preservative could experience respiratory discomfort, headaches and nausea, the department's spokesman said.

Sulfur dioxide was of low toxicity and commonly used in the food manufacturing industry as a preservative, he said.